For the slightly longer version...
A field code is like a little program that tells Word what to display as the
field's result. If the code says IncludePicture followed by the name of a
graphic file, then the result will be the contents of that file. If the code
says Date followed by some formatting information, the result will be
today's date in that format. There are dozens of kinds of fields. Every
field has a code, whether you can see it or not. (Some codes never produce
any visible result.)
There are several ways to make the codes visible/invisible. If you press
Alt+F9 repeatedly, all the fields in the document switch from codes to
results or vice versa. This is equivalent to going to the Tools > Options >
View dialog and checking/unchecking the Field Codes box.
If you select a particular field and press Shift+F9, only that one field
switches back and forth.
Finally, a few field codes (TC for table of contents entries, XE for index
entries, and I think one or two more) are automatically formatted with the
Hidden font format, so you have to display hidden text to see them; Alt+F9
and Shift+F9 have no effect on them.